This invention has been designed to alleviate flutter and suppress vibration of stores suspended from a support such as an aircraft wing.
Flutter is a dangerous aerodynamic instability which affects lifting surfaces in a fluid flow. Flutter speed is the speed at which a given aircraft will begin to experience these self-induced oscillations. The vibrations which characterize flutter have potentially catastrophic results; aircraft have literally broken apart because of flutter.
Classical bending-torsion flutter involves coupling of at least two natural vibration modes, one or more of which contain torsional deformations of the wing. The frequencies of bending and torsion modes vary with airspeed and couple as flutter is approched.
Current fighter/attack aircraft are required to carry a vast number of external, wing or fuselage mounted stores. With multiple store attachment locations on the wing, each designed to accommodate an array of store configurations, some having variable mass (e.g., fuel tanks and rocket pods), there are literally thousands of possible store loading combinations for a single aircraft. The attachment of a store mass to a wing alters the dynamic characteristics of the structure and often causes drastic reductions in the flutter speed, which can result in catastrophic structural failures. Extensive and costly effort in the form of mathematical analyses, wind tunnel mode tests, and flight flutter tests are performed to assure safety from flutter.
For flutter critical store configurations either the flutter speed must be raised by some means or restrictions placed on the aircraft operating envelope. The flutter speed can be raised by conventional passive methods or by more advanced methods involving active flutter suppression. Some examples of passive methods are: adding mass ballast, tuning the store pylon stiffness characteristics or relocating the wing store attachment point. Passive schemes of this kind are generally tailored for a specific store configuration and are not readily changed to accommodate the necessary broad range of store mass and inertia combinations.
Active flutter suppression concepts have been the subject of considerable research in recent years. In this approach the flutter mode response is sensed by a transducer whose electrical output is modified by an appropriate control law and fed back to a control surface actuator to produce an aerodynamic force opposing flutter. Compared with passive methods, active control of flutter has the advantage of possible weight savings plus versatility gains. Although active control of wing store flutter has been successfully demonstrated in wind tunnel tests and in flight, there are drawbacks which hinder its use in practical applications. Among these drawbacks are: (1) need for accurate knowledge of unsteady aerodynamic control forces, particularly at transonic speeds where flutter is most likely to occur and theory is least developed; (2) need for high-power, fast acting control systems; and (3) marginal ability to increase flutter speed in cases of violent-type flutter.
A particular active flutter suppression concept investigated by Triplett et al. and described in "Active Flutter Suppression Systems for Military Aircraft--A Feasibility Study", AFFDL-TR-72-116, Feb. 1973, uses hydraulic actuators as the load carrying tie between wing and store. Through feedback control, the actuators nullify dynamic loads but transmit steady loads to the wing. By dynamically decoupling the wing/store system in this way, the flutter mechanism and speed revert to that of the bare wing. Unfortunately, this potentially promising scheme for wing/store flutter control was found to be impractical due to excessive flow rates required by the actuators.
Some research in helicopter design has focused on carrier pod alignment and vibration transfer. Active alignment systems as described in U.S. Pat. No. 3,904,156, include sensors which, when actuated by angular displacement of the helicopter pod, electronically initiate motion of the load arms to damp the displacement. One passive vibration isolator, described in U.S. Pat. No. 3,176,939, adds a pneumatic spring at each point of attachment of the pod to the carrier. These springs, tuned to be soft (with little resistance to external force), correspond to the tuned pylon arrangements seen in wing-store flutter suppression.
The need remains for an effective means of either compensating for or reducing the flutter burden placed on an aircraft wing by an attached store, allowing realignment of the store with the wing after angular displacements. The decoupler pylon described herein alleviates wing-store flutter using elements of both active and passive suppression. Bending and torsion mode frequencies are separated by the arrangement, thus increasing flutter speed without performance penalties.
An object of the present invention, then, is to provide means for suppressing wing-store flutter for all flight conditions within the aircraft's design envelope.
Another object of the present invention is to provide an attachment, the use of which will reduce the amount of flutter testing and analysis now necessary for aircraft that accommodate a large number of wing mounted store configuration.
Another object of the present invention is to provide an attachment, which can make flutter speed insensitive to variations in center of gravity and store inertia properties which may change during flight.
Another object of the present invention is to provide an arrangement wherein aspects of both passive and active flutter suppression may be employed.
Yet another object of the present invention is to provide an attachment wherein soft spring/damper elements decouple store pitch motions from the wing.
Another object of the present invention is to provide an attachment, whereby the store is isolated from shock and vibration loads, such as buffeting, induced by the wing or other support structure.
Still another object of the present invention is to provide an attachment wherein a low-power control automatically aligns the store with the wing under conditions of changing mean load.